There are several reasons for needing accurate time information in a mobile terminal, but of particular interest here is the need to support a satellite positioning receiver. For example, in a mobile telephone of a communications system, a GPS receiver may be used to provide the position of the terminal to an emergency service, or for access to some other location-based service. The problem encountered in such circumstances is that the mobile telephone may well be inside a building or other shielded space where the satellite signals are attenuated detrimentally. The GPS receiver then usually has to perform lengthy searches involving long integration times to acquire the signals from each satellite in view, resulting in an unacceptable delay before the position becomes available or the acquisition process fails. The delay can be shortened in GPS receivers having access to accurate time and/or frequency information as the search window in time and/or frequency can then be narrowed, requiring fewer correlations and shorter searches. If the time and/or frequency information is sufficiently accurate, it may be possible to identify the offsets at which to integrate with sufficient accuracy that the GPS receiver can be run in re-acquisition mode, thereby reducing the length of time that the receiver needs to be switched on and hence reducing the drain on the terminal's computation and power resources.
One of the ways of providing the time information is to use the signals from the communications network. In WO 2005/071430, it is shown how a network using technology such as GSM or W-CDMA can be regarded as a stable, but unsynchronized, repository of time. A calibration is first performed in the mobile terminal using the GPS receiver to obtain a full position-and-time fix. The terminal also has a database of the network transmitter positions and is able to compute the times of flight from each transmitter to the terminal. Furthermore, although the network may be unsynchronised so that the transmissions have arbitrary time offsets, the terminal has access to, or is able to maintain, a list of these transmission time offsets which may, for example, be determined using the method as taught in WO 00/73813 and WO 00/73814. The calibration therefore establishes the transmission time offsets with respect to GPS time. At a later instant, for example when the terminal has moved inside a building and the GPS signals are attenuated, the GPS time may be obtained from the receipt of signals from the network transmitters together with the calibration values. This provides a robust method of providing time assistance to the GPS receiver which is maintained through periods in which the mobile terminal may be turned off and which, in a GSM system, can be accurate at the sub-microsecond level.
A problem encountered with the method taught in WO 2005/071430 is that the terminal needs to have a database of transmitter positions, and may also need to know the operational frequency plan, that is a plan of the radio-frequency channels in use by each base station together with the directional characteristics of the antennas. We have realised that in some circumstances, and with reduced accuracy, it is possible to measure the time interval between one instant and another in the terminal using the received network signals without any knowledge of where they come from, and without needing to know their transmission time offsets. Such a measured time interval is not related directly to a universal time in that no calibration step has been carried out. The interval is measured with respect to an average of the clocks controlling the network transmitters, which may be running fast or slow, and which may be independent of each other. Nevertheless, in networks controlled by sufficiently-stable oscillators (as is usually the case in practice), this ‘average network time’ interval may be sufficiently close to a universal time interval (such as one measured using GPS time) to be useful.